1. Field of the Invention
This invention relates to a scanning optical apparatus usable in an image forming apparatus such as a laser beam printer or a digital copier.
2. Description of the Related Art
A scanning optical apparatus of this kind is constructed as shown, for example, in FIG. 13 of the accompanying drawings, and in the interior of a frame 1 made of synthetic resin, there are disposed a light source unit 2, a cylinder lens 3, a deflecting and scanning unit 4, scanning lenses 5a, 5b, a turn-back mirror 6, a light detecting unit 7, a reflecting mirror 8, etc. The frame 1 has a peripheral wall 1a for preventing a laser beam from emerging to the outside, and a plurality of mounting holes 1b for fixing means, and the upper opening of the frame 1 is adapted to be closed by a lid.
Such a scanning optical apparatus, as shown in FIG. 14 of the accompanying drawings, is mounted for use on an image forming apparatus as a host apparatus. The frame 1 of the scanning optical apparatus and the frame F of the image forming apparatus are fixed to each other by screws 9 inserted in the mounting holes 1b. A process cartridge P provided with a photosensitive drum D is incorporated in the frame of the image forming apparatus so as to be located below the scanning optical apparatus.
The laser beam emitted from the light source unit 2 is transmitted through the cylinder lens 3, is deflected by the deflecting and scanning unit 4, is transmitted through the scanning lenses 5a and 5b, and thereafter is reflected by the turn-back mirror 6, and emerges from the emergence port 1c of the frame 1 toward the photosensitive drum D and forms a latent image on the surface of the photosensitive drum D.
Also, before it emerges toward the photosensitive drum D, the laser beam is reflected by the reflecting mirror 8 upstream with respect to the main scanning direction and enters the light detecting unit 7. An electrical signal from the light detecting unit 7 is adapted to determine the timing of the start of image recording, and the image forming apparatus receives the input of the electrical signal from the light detecting unit 7, and forms and outputs an image in operative association with the scanning optical apparatus.
In the conventional scanning optical apparatus, however, the mounted shapes of optical parts are diversified and therefore, the shape of the frame 1 is complicated. Also, the frame F of the conventional image forming apparatus limits the locations of the mounting holes 1b of the scanning optical apparatus and maldistributes them relative to the configuration of the scanning optical apparatus, in order to secure a space for containing the process cartridge P therein. Therefore, in the frame 1 after the scanning optical apparatus is incorporated into the image forming apparatus, there exist a fixed portion 1A surrounded by the mounting holes 1b and fixed to the image forming apparatus, and an overhanging portion 1B of cantilever structure overhanging from this fixed portion 1A into the air.
When the deflecting and scanning unit 4 is operated in such a state, the frame 1 vibrates in response to the vibration of this deflecting and scanning unit 4. In this case, the vibration occurring in each portion of the frame 1 does not become uniform because the mounted shapes of the optical parts are diversified and the shape of the frame 1 is complicated. Accordingly, vibrations from multiple directions are combined in the frame 1 and for example, a localized resonance phenomenon occurs to the protruding portion. That is, the overhanging portion 1B of the frame 1 overhangs as cantilever structure and therefore, resonance occurs there at a relatively low frequency.
Specifically, the overhanging portion 1B of the frame 1 is great in volume and weight and moreover is of cantilever structure and therefore may in some cases cause resonance at a relatively low frequency, e.g. 500 Hz or less. In contrast, the rotational frequency of the deflecting and scanning unit 4 often exceeds 300 Hz with the tendency toward a higher speed in recent years, and may often approximate to the resonance frequency of the overhanging portion 1B of the frame 1.
Accordingly, when resonance occurs in the overhanging portion 1A at a frequency approximate to the rated rotation of the deflecting and scanning unit 4, the vibration of the scanning optical apparatus increases during print output and at the same time, great vibration also propagates to the image forming apparatus. This vibration deviates the relative relation between the position of the laser beam emitted from the scanning optical apparatus and the position of the photosensitive drum D positioned in the frame, causes uneven pitch to a print-outputted image and deteriorates the quality of the image.
It is an object of the present invention to provide a scanning optical apparatus in which the vibration of the overhanging portion of a frame is suppressed to thereby improve the quality of image.
It is another object of the present invention to provide a scanning optical apparatus comprising a light source emitting a light beam, deflecting and scanning means for deflecting and scanning the light beam emitted from the light source, a frame body provided with a side wall and containing the light source and the deflecting and scanning means therein, and a supporting member for supporting the frame body, the frame body having a fixed portion fixed to and supported by the supporting member, and an overhanging portion overhanging from the fixed portion, the side wall having a cut-away portion between the fixed portion and the overhanging portion.
Further objects of the present invention will become apparent from the following description.